one-line percussion staff: barlines end incorrectly

[Antti Kaihola]

One-line drum staves work better now, but there's still something wrong
with the barlines and especially repeats.

I'd like the barlines to extend above the single staff line. Imagine
that it's really a 5-line staff with the top and bottom two lines
hidden. Or is it already possible to hide staff lines without it
affecting anything else?

Here's my test case:

\context StaffGroup <<
    \new DrumStaff \with { \override StaffSymbol #'line-count = #1 }
    \drummode { mar2 mar | mar4 mar mar mar | \repeat volta 2 { mar2 mar | } }
    \new Staff \relative { c4 d e f | g a b c | \repeat volta 2 { b a g f } }
>>